This may be an impossible task, but I'm trying to identify an old tube. Here's the deal... I bought this homemade amp that looks like it was built using transformers and some caps from an early 50s RCA radio perhaps. It has a 25L6GT and a 6AV6... and 3 small unidentified tubes. There's a bit of writing left on one and it looks like it may have said something like 6AK5W... but the A is the only firmly legible thing. First number may be a 3 or 8. Is there any way to guess what they may be? The person I bought it from suggested 6AH5... but I don't know if that was an educated guess or just an attempt to read it. Thanks for any help...

Assuming the original tube has a good heater feed it 6 volts and see if it lights up.

The tube has the guts of a 6AK5 or a 18AK5 which was very uncommon in the US. The only evidence that they exist comes from Germany and the tubes were Siemens. There are a few other tubes that look the same 6AS6 and 6AJ5 come to mind. The 6AH5 is a large octal tube, also quite rare.

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